Differential transmission circuit

ABSTRACT

A differential transmission circuit comprises a sending unit that generates a pair of differential signals from an input signal, and sends the differential signals; a receiver that receives the differential signals sent by the sending unit; and a transmission path that transmits the differential signals from the sending unit to the receiver, wherein the sending unit has a selector that selects one of the input signal and a signal obtained by inverting a polarity of the input signal, and generates the differential signals from the signal selected by the selector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/651,514 filed Oct. 15, 2012, which is a continuation of U.S. Pat. No.8,310,276 filed Dec. 21, 2010, which claims the benefit of and priorityto Japanese Patent Application No. 2008-169332 filed Jun. 27, 2008 andJapanese Patent Application No. 2008-204773 filed Aug. 7, 2008, each ofwhich are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit configuration fortransmitting differential signals.

2. Description of the Related Art

The need to transmit high frequency digital signals has arisen with thedigitization and increasing speed of electronic devices. Radiated noisenot only from external interfaces and transmission cables between boardsin a device but also from wiring between IC chips on the same board hasthus become a significant problem.

A differential transmission system is widely used as a countermeasure tothis radiated noise, although with a differential transmission system,the wirings of transmission paths intersect due to the terminalarrangement of the IC chips, resulting in distortion of characteristicimpedance. If appropriate wiring that does not intersect is to beperformed, IC chips having the same function but different terminalarrangements will thus be required.

In Japanese Patent Laid-Open No. 2007-149805, the configuration of aprinted wiring board that maintains isometricity and reduces distortionof characteristic impedance even in the case where wirings forperforming differential transmission intersect is disclosed.

In Japanese Patent Laid-Open No. 2003-249904, an integrated circuithaving a shield effect that protects transmission signals fromelectromagnetic interference by enclosing connection balls with thepower supply and GND is disclosed.

In Japanese Patent Laid-Open No. 11-186674, a technique that preventsloss of signal quality and radiation of noise by arranging a pair ofdata transmission circuits axisymmetrically on the board of adifferential transmission circuit is disclosed.

DISCLOSURE OF INVENTION

FIG. 14 shows a configuration of a conventional differentialtransmission circuit board.

In FIG. 14, reference numeral 1400 denotes a sending unit that sends apair of differential signals. Reference numeral 1410 denotes a receiverthat receives the differential signals sent by the sending unit 1400.The sending unit 1400 and the receiver 1410 are respectively providedwith BGA (Ball Grid Array) P (positive logic) terminals 1401 and 1411and N (negative logic) terminals 1402 and 1412. Reference numeral 1420denotes transmission paths for transferring the differential signals. Inthe case where the sending unit 1400 and the receiver 1410 areimplemented on the same surface of a board 1480, the transmission paths1420 are isometric and parallel.

However, in the case where a sending unit 1500 and a receiver 1510 areimplemented on different surfaces, such as the front and the back, of aboard 1580 as shown in FIG. 15, the positions of a P terminal 1512 andan N terminal 1511 of the receiver 1510 are inverted with respect to a Pterminal 1501 and an N terminal 1502 of the sending unit 1500. Thus,transmission paths 1520 intersect, causing mismatching of characteristicimpedance and loss of signal quality. The dotted lines in FIG. 15 showthat the receiver 1510 is implemented on the surface on the oppositeside of the board 1580 to the sending unit 1500.

The appearance of a circuit board when the sending unit and the receiverare implemented on the same surface of the board in a differentialtransmission system for transmitting a plurality of signals is shown inFIG. 16.

In FIG. 16, reference numeral 1600 denotes a sending unit that sendsdifferential signals. Reference numeral 1610 denotes a receiver thatreceives the differential signals sent by the sending unit 1600.Reference numeral 1620 denotes transmission paths for transferring thedifferential signals. Reference numerals 1601 and 1602 denote inputsignals for generating the differential signals. Reference numerals 1603and 1604 denote flip-flop circuits for respectively holding the Data1and Data2 input signals 1601 and 1602. Reference numerals 1605 and 1606denote buffers for respectively generating the differential signals fromthe input signals 1601 and 1602. Reference numerals 1615 and 1616 denotebuffers that receive the differential signals sent via the transmissionpaths 1620 and output differences. Reference numerals 1611 and 1612denote output signals respectively generated by the buffers 1615 and1616. Reference numerals 1613 and 1614 denote flip-flop circuits forrespectively holding the output signals 1611 and 1612. The differentialsignals are respectively generated by the buffers 1605 and 1606 from theinput signals 1601 and 1602, and sent to the receiver 1610 via thetransmission paths 1620. The flip-flop circuits 1613 and 1614 hold theoutput signals 1611 and 1612 generated by the buffers 1615 and 1616 fromthe received differential signals.

The appearance of a circuit board when the sending unit and the receiverare implemented on different surfaces of the board is shown in FIG. 17.

In FIG. 17, reference numeral 1780 denotes a circuit board, and 1781denotes via holes. In this case, the signals get switched around in thesending unit 1700 and the receiver 1710 as shown in FIG. 17 whentransmission paths 1720 are kept isometric and parallel. If the circuitboard is configured so that the signals are not switched around,transmission paths 1820 intersect as shown in FIG. 18, making itdifficult to control the characteristic impedance of the transmissionpaths.

Here, with the above-mentioned Japanese Patent Laid-Open No.2007-149805, when a plurality of pairs of differential signals areprovided in the case where the input unit and the output unit areimplemented on different surfaces of the board, the wirings of thetransmission paths for the signal pairs intersect, and theabove-mentioned problems cannot be addressed.

With the above-mentioned Japanese Patent Laid-Open No. 2003-249904, theconnection balls are arranged symmetrically on the board with respect toa perpendicular surface, but when the sending unit and the receiver areimplemented on different surfaces of the board in differentialtransmission, the transmission paths intersect and characteristicimpedance will be mismatched. Thus, the problem of signal quality losscannot be solved.

Further, loss of signal quality due to distortion of characteristicimpedance when wires intersect cannot be addressed simply by arrangingthe transmission paths for differential signals axisymmetrically as inJapanese Patent Laid-Open No. 11-186674.

The present invention has been made in consideration of theabove-mentioned problems, and realizes a circuit configuration thatenables a sending unit and a receiver to be implemented on differentsurfaces of a board with IC chips having the same terminal arrangement,while keeping transmission paths in a differential transmission systemparallel and isometric.

In order to solve the aforementioned problems, the present inventionprovides a differential transmission circuit comprising: a sending unitthat generates a pair of differential signals from an input signal, andsends the differential signals; a receiver that receives thedifferential signals sent by the sending unit; and a transmission paththat transmits the differential signals from the sending unit to thereceiver, wherein the sending unit has a selector that selects one ofthe input signal and a signal obtained by inverting a polarity of theinput signal, and generates the differential signals from the signalselected by the selector.

The present invention also provides a differential transmission circuitcomprising: a sending unit that generates a pair of differential signalsfrom an input signal, and sends the differential signals; a receiverthat receives the differential signals sent by the sending unit; and atransmission path that transmits the differential signals from thesending unit to the receiver, wherein the receiver has a selector thatselects one of a difference signal obtained by taking a difference ofthe differential signals and a signal obtained by inverting a polarityof the difference signal, and generates an output signal from the signalselected by the selector.

The present invention also provides a differential transmission circuitcomprising: a sending unit that generates a pair of differential signalsfrom an input signal, and sends the differential signals from a Pterminal and an N terminal; a receiver that receives, at a P terminaland an N terminal, the differential signals sent by the sending unit,and generates an output signal; a first transmission path that transfersa differential signal from the P terminal of the sending unit to the Pterminal of the receiver; and a second transmission path that transfersa differential signal from the N terminal of the sending unit to the Nterminal of the receiver, wherein the P terminal and the N terminal ofat least one of the sending unit and the receiver are lined up parallelto the first transmission path and the second transmission path.

According to the present invention, a sending unit and a receiver can beimplemented on different surfaces of a board with IC chips having thesame terminal arrangement, while keeping transmission paths in adifferential transmission system parallel and isometric.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a board configuration in the case where a differentialtransmission circuit of a first embodiment according to the presentinvention is implemented on the same surface of a board.

FIG. 2 shows a board configuration in the case where a differentialtransmission circuit of the first embodiment according to the presentinvention is implemented on different surfaces of a board.

FIG. 3 shows waveforms of signals up until an output signal is obtainedin a differential transmission circuit of the first embodiment.

FIG. 4 shows a board configuration in the case where a differentialtransmission circuit of a second embodiment is implemented on the samesurface of a board.

FIG. 5 shows a board configuration in the case where a differentialtransmission circuit of the second embodiment is implemented ondifferent surfaces of a board.

FIG. 6 shows a board configuration in the case where a differentialtransmission circuit of a third embodiment is implemented on the samesurface of a board.

FIG. 7 shows a board configuration in the case where a differentialtransmission circuit of the third embodiment is implemented on differentsurfaces of a board.

FIG. 8 shows a configuration of a sending unit of a differentialtransmission circuit.

FIG. 9 shows a board configuration in the case where a differentialtransmission circuit of a fourth embodiment is implemented on the samesurface of a board.

FIG. 10 shows signal waveforms obtained by a differential transmissioncircuit of the fourth embodiment.

FIG. 11 shows a board configuration in the case where a differentialtransmission circuit of the fourth embodiment is implemented ondifferent surfaces of a board.

FIG. 12 shows a board configuration in the case where a differentialtransmission circuit of a fifth embodiment is implemented on the samesurface of a board.

FIG. 13 shows a board configuration in the case where a differentialtransmission circuit of the fifth embodiment is implemented on differentsurfaces of a board.

FIG. 14 shows a board configuration in the case where a differentialtransmission circuit of conventional technology is implemented on thesame surface of a board.

FIG. 15 shows a board configuration in the case where a differentialtransmission circuit of conventional technology is implemented ondifferent surfaces of a board.

FIG. 16 shows a board configuration in the case where a differentialtransmission circuit of conventional technology is implemented on thesame surface of a board.

FIG. 17 shows a board configuration in the case where a differentialtransmission circuit of conventional technology is implemented ondifferent surfaces of a board.

FIG. 18 shows a circuit configuration for preventing signals from beingswitched around in the differential transmission circuit of FIG. 17.

FIG. 19 shows a board configuration in the case where a differentialtransmission circuit of a sixth embodiment according to the presentinvention is implemented on the same surface of a board.

FIG. 20 shows a board configuration in the case where a differentialtransmission circuit of the sixth embodiment according to the presentinvention is implemented on different surfaces of a board.

FIG. 21 shows a board configuration in the case where a differentialtransmission circuit of the sixth embodiment according to the presentinvention is implemented on different surfaces of a board, and Pterminals and N terminals are lined up in different directions, parallelto transmission paths.

FIG. 22 shows a board configuration in the case where a differentialtransmission circuit of the sixth embodiment according to the presentinvention is implemented on different surfaces of a board, and Pterminals and N terminals are lined up in different directions so as tobe orthogonal to transmission paths.

DESCRIPTION OF THE EMBODIMENTS

Embodiments for implementing the present invention will now be describedin detail with reference to the accompanying drawings.

Note that the embodiments to be described hereinafter are merelyexamples for realizing the present invention, and should beappropriately modified or changed depending on the configuration of theapparatus to which the present invention is applied and variousconditions. Moreover, the present invention is not limited to thefollowing embodiments.

First Embodiment

FIG. 1 shows a configuration of a differential transmission circuitboard of a first embodiment according to the present invention.

In FIG. 1, reference numeral 100 denotes a sending unit that sends apair of differential signals, and 110 denotes a receiver that receivesthe differential signals sent by the sending unit 100. Reference numeral120 denotes transmission paths for transferring the differentialsignals. Reference numeral 101 denotes an input signal for generatingthe differential signals, and 102 denotes a flip-flop circuit forholding the input signal 101. Reference numeral 103 denotes an inverterfor inverting the polarity of the input signal 101, and 104 denotes aselector that selects one of the input signal 101 and the signal fromthe inverter 103. Reference numeral 105 denotes a buffer that generatesthe differential signals from the signal selected by the selector 104,and 115 denotes a buffer that receives the differential signalsgenerated by the buffer 105 and sent via the transmission paths 120, andoutputs a difference (difference signal). Reference numeral 112 denotesa flip-flop circuit for holding an output signal 111 generated by thebuffer 115. Reference numeral 180 denotes a circuit board.

The waveforms of the signals up until the output signal 111 is obtainedare shown in FIG. 3. In FIG. 3, a, b, c, d and e show the signals on thewirings in FIG. 1. In FIG. 3, H and L respectively show the high leveland low level of the signals. The dotted line 300 is a common modevoltage of the differential signals.

In the case where the sending unit 100 and the receiver 110 areimplemented on the same surface of the board, the transmission paths 120are isometric and parallel. In this case, the selector 104 selects theinput signal 101 from the flip-flop circuit 102. Differential signalsgenerated by the buffer 105 from the signal selected by the selector 104are sent from the sending unit 100 to the receiver 110 via thetransmission paths 120. The output signal 111 generated by the buffer115 from the received differential signals is held in the flip-flopcircuit 112. Assuming that the input signal 101 was a signal with awaveform such as the signal a, the signal b selected by the selector 104will also have the same waveform. The differential signals generated bythe buffer 105 will be respectively like the signals c and d, and theoutput signal 111 generated in the buffer 115 from the difference c-dwill have a waveform such as the signal e.

The appearance of a circuit board when the sending unit and the receiverare implemented on different surfaces of the board is shown in FIG. 2.

In FIG. 2, reference numeral 280 denotes a circuit board, and 281denotes via holes. The polarity of a buffer 215 is switched in order tokeep transmission paths 220 parallel and isometric. In this case, aselector 204 selects the signal obtained by inverting the polarity of aninput signal 201 from a flip-flop circuit 202 in an inverter 203.Differential signals generated by the buffer 205 from the signalselected by the selector 204 are sent from the sending unit 200 to areceiver 210 via the transmission paths 220. An output signal 211generated by the buffer 215 from the received differential signals isheld in a flip-flop circuit 212. Assuming that the input signal 201 wasa signal with a waveform such as the signal a, the signal b selected bythe selector 204 will have a waveform inverted by the inverter 203. Thedifferential signals generated by the buffer 205 will be respectivelylike the signals c and d, and the output signal 211 generated in thebuffer 215 from the difference d-c will have a waveform such as thesignal e. The same output signal as when the sending unit and thereceiver are implemented on the same surface is thus obtained.

According to the present embodiment, a differential transmission circuitthat is compatible in the case where the sending unit and the receiverare implemented either on the same surface or on different surfaces of aboard with IC chips having the same terminal arrangement can be realizedwhen transmitting a signal using a differential transmission system.This differential transmission circuit is also compatible with boardarea reduction and device miniaturization.

Second Embodiment

FIG. 4 shows a configuration of a differential transmission circuitboard of a second embodiment.

In FIG. 4, reference numeral 400 denotes a sending unit that sendsdifferential signals, and 410 denotes a receiver that receives thedifferential signals sent by the sending unit 400. Reference numeral 420denotes transmission paths for transferring the differential signals.Reference numeral 401 denotes an input signal for generating thedifferential signals, and 402 denotes a flip-flop circuit for holdingthe input signal 401. Reference numeral 405 denotes a buffer forgenerating the differential signals from the input signal 401, and 415denotes a buffer that receives the differential signals generated by thebuffer 405 and sent via the transmission paths 420, and outputs adifference (difference signal). Reference numeral 413 denotes aninverter for inverting the polarity of the signal output by the buffer415, and 414 denotes a selector that selects one of the signal from thebuffer 415 and the signal from the inverter 413. Reference numeral 412denotes a flip-flop circuit for holding an output signal 411 selected bythe selector 414. Reference numeral 480 denotes a circuit board.

In the case where the sending unit 400 and the receiver 410 areimplemented on the same surface of the board, the differential signalsare generated from the input signal 401 and sent from the sending unit400 to the receiver 410 via the transmission paths 420. The selector 414selects the signal generated by the buffer 415 from the receiveddifferential signals, and the selected signal is held in the flip-flopcircuit 412.

The appearance of a circuit board when the sending unit and the receiverare implemented on different surfaces of the board is shown in FIG. 5.

In FIG. 5, reference numeral 580 denotes a circuit board, and 581denotes via holes. The polarity of a buffer 515 is switched in order tokeep transmission paths 520 parallel and isometric. In this case, as aresult of a selector 514 selecting a signal generated by the buffer 515from received differential signals and polarity inverted in an inverter513, the same output signal as when the sending unit and the receiverare implemented on the same surface is obtained and held in a flip-flopcircuit 512.

According to the present embodiment, a differential transmission circuitthat is compatible in the case where the sending unit and the receiverare implemented either on the same surface or on different surfaces of aboard with IC chips having the same terminal arrangement can be realizedwhen transmitting a signal using a differential transmission system.This differential transmission circuit is also compatible with boardarea reduction and device miniaturization.

Third Embodiment

FIG. 6 shows a configuration of a differential transmission circuitboard of a third embodiment.

In FIG. 6, reference numeral 600 denotes a sending unit that sendsdifferential signals. Reference numeral 620 denotes a receiver thatreceives the differential signals sent by the sending unit 600.Reference numerals 640 a and 640 b denote transmission paths (firsttransmission paths, second transmission paths) for transferring thedifferential signals. Reference numerals 601 and 602 denote inputsignals (first input signal, second input signal) for generating thedifferential signals (first differential signals, second differentialsignals). Reference numerals 610 and 611 denote flip-flop circuits forrespectively holding the Data1 and Data2 input signals 601 and 602.Reference numerals 612 and 613 denote inverters for inverting therespective polarities of the input signals 601 and 602. Referencenumeral 614 denotes a selector (first selector) that selects one of theinput signal 601 and the signal from the inverter 612. Reference numeral615 denotes a selector (second selector) that selects one of the inputsignal 602 and the signal from the inverter 613. Reference numerals 616and 617 denote selectors (third selector, fourth selector) thatrespectively select one of the signal from the selector 614 and thesignal from the selector 615 as a third signal and a fourth signal.Reference numerals 618 and 619 denote buffers for respectivelygenerating differential signals from the signals selected by theselectors 616 and 617. Reference numerals 632 and 633 denote buffers forrespectively receiving the signals generated by the buffers 618 and 619and sent via the transmission paths 640 a and 640 b, and outputtingdifferences (difference signals). Reference numerals 621 and 622respectively denote output signals (first output signal, second outputsignal) generated by the buffers 632 and 633. Reference numerals 630 and631 denote flip-flop circuits for respectively holding the outputsignals 621 and 622.

In the case where the sending unit 600 and the receiver 620 areimplemented on the same surface of the board, the selectors 614 and 615respectively select the input signals 601 and 602. The selector 616selects the signal from the selector 614, and the selector 617 selectsthe signal from the selector 615. Differential signals generated by thebuffers 618 and 619 from the selected signals are respectively sent fromthe sending unit 600 to the receiver 620 via the transmission paths 640a and 640 b. The output signals generated by the buffers 632 and 633from the received differential signals are respectively held in theflip-flop circuits 630 and 631.

FIG. 7 shows the appearance of a circuit board when the sending unit andthe receiver are implemented on different surfaces of the board.

In FIG. 7, reference numeral 780 denotes a circuit board, and 781denotes via holes. The polarities of buffers 732 and 733 are switched inorder to keep both transmission paths 740 a and 740 b parallel andisometric. In this case, selectors 714 and 715 respectively selectsignals obtained by inverting the polarities of input signals 701 and702 in inverters 712 and 713. The Data1 and Data2 input signals are thenswitched as a result of a selector 716 selecting the signal from theselector 715 and a selector 717 selecting the signal from the selector714. Differential signals respectively generated by buffers 718 and 719from the signals selected by the selectors 716 and 717 are sent to areceiver 720 via the transmission paths 740 a and 740 b. Output signals721 and 722 the same as when the sending unit and the receiver areimplemented on the same surface are obtained by the buffers 732 and 733from the received differential signals, and respectively held inflip-flop circuits 731 and 730. Note that while the case where twosignals are transmitted has been described, the present invention isalso applicable in the case where three or more signals are transmitted.

According to the present embodiment, a differential transmission circuitthat is compatible in the case where the sending unit and the receiverare implemented either on the same surface or on different surfaces of aboard with IC chips having the same terminal arrangement can be realizedwhen transmitting a plurality of signals using a differentialtransmission system. This differential transmission circuit is alsocompatible with board area reduction and device miniaturization.

Fourth Embodiment

With differential transmission for transmitting a plurality of signals,a mechanism such as shown in FIG. 8, for example, for adjusting a clockthat clocks an upstream flip-flop is required in the sending unit inorder to synchronize the output timing, in the case where one of theplurality of input signals is a clock signal.

In FIG. 8, reference numeral 800 denotes the configuration of a sendingunit in a differential transmission circuit. Reference numeral 801denotes a DDR (Double Data Rate) clock. Reference numeral 802 denotes aninverter that inverts the clock 801. Reference numerals 803 and 804denote flip-flop circuits for respectively holding Data and Clk inputsignals for differential transmission. Reference numerals 805 and 806denote buffers for respectively generating differential signals from thesignals from the flip-flop circuits 803 and 804.

FIG. 9 shows a configuration of a differential transmission circuitboard of the fourth embodiment.

In FIG. 9, reference numeral 900 denotes a sending unit that sendsdifferential signals. Reference numeral 920 denotes a receiver thatreceives the differential signals sent by the sending unit 900.Reference numerals 940 a and 940 b denote transmission paths (firsttransmission paths, second transmission paths) for transferring thedifferential signals. Reference numerals 901 and 902 denote inputsignals (first input signal, second input signal) for generating thedifferential signals. Reference numerals 910 and 911 denote flip-flopcircuits for respectively holding the Data and Clk input signals 901 and902. Reference numerals 912 and 913 denote buffers for respectivelygenerating differential signals (first differential signals, seconddifferential signals) from the input signals 901 and 902. Referencenumerals 930 and 931 denote buffers for respectively receiving thedifferential signals sent via the transmission paths 940 a and 940 b,and outputting differences (first difference signal, second differencesignal). Reference numerals 932 and 933 denote inverters forrespectively inverting the polarities of the signals from the buffers930 and 931. Reference numeral 934 denotes a selector (first selector)that selects one of the signal from the buffer 930 and the signal fromthe inverter 932. Reference numeral 935 denotes a selector (secondselector) that selects one of the signal from the buffer 931 and thesignal from the inverter 933. Reference numerals 936 and 937 denoteselectors (third selector, fourth selector) that respectively select oneof the signal from the selector 934 and the signal from the selector 935as a third signal and a fourth signal. Reference numeral 921 denotes anoutput signal generated by the selector 936. Reference numeral 922denotes an output signal obtained by doubling the frequency of thesignal from the selector 937. Reference numeral 938 denotes a flip-flopcircuit for holding the output signal 921. Reference numeral 939 denotesa PLL (Phase Locked Loop) circuit for multiplying the signal frequency.

Here, the case where the sending unit and the receiver are implementedon the same surface of the board will be described.

Differential signals are generated by the buffers 912 and 913 from theinput signals 901 and 902 and sent to the receiver 920 via thetransmission paths 940 a and 940 b. The selectors 934 and 935 select thesignals generated by the buffers 930 and 931 from the receiveddifferential signals. The selector 936 selects the signal from theselector 934, and the selector 937 selects the signal from the selector935. The output signal 921 from the selector 936 is held in theflip-flop circuit 938, and the frequency of the output signal 922 fromthe selector 937 is doubled by the PPL circuit 939. The waveformsthereby obtained are shown in FIG. 10. Reference numerals 1001 and 1002respectively denote the Data signal and the Clk signal to be sent.Reference numeral 1022 denotes a Clk signal obtained as a result of thefrequency of the received Clk signal being doubled by the PPL circuit.Reference numeral 1021 denotes the received Data signal.

The appearance of a circuit board when the sending unit and the receiverare implemented on different surfaces of the board is shown in FIG. 11.

In FIG. 11, reference numeral 1180 denotes a circuit board, and 1181denotes via holes. In this case, the polarities of buffers 1130 and 1131are switched in order to keep both transmission paths 1140 a and 1140 bparallel and isometric. Differential signals are generated by buffers1112 and 1113 from input signals 1101 and 1102, and sent to a receiver1120 via the transmission paths 1140 a and 1140 b. Selectors 1134 and1135 respectively select signals generated by buffers 1131 and 1130 fromreceived differential signals and polarity inverted in inverters 1132and 1133. The Data signal and the Clk signal are then switched as aresult of a selector 1136 selecting the signal from the selector 1135,and a selector 1137 selecting the signal from the selector 1134. Outputsignals 1122 and 1121 from the selectors 1136 and 1137 are the sameoutput signals as when the sending unit and the receiver are implementedon the same surface, with the output signal 1121 being held in theflip-flop circuit 1138, and the frequency of the output signal 1122being doubled by the PLL circuit 1139. Note that while the case wherethe transmission signals are a single Data signal and Clk signal hasbeen described, the present invention is also applicable in the casewhere two or more Data signals and Clk signals are transmitted.

According to the present embodiment, a differential transmission circuitthat is compatible in the case where the sending unit and the receiverare implemented either on the same surface or on different surfaces of aboard with IC chips having the same terminal arrangement can be realizedwhen transmitting a plurality of signals that include a clock signalusing a differential transmission system. This differential transmissioncircuit is also compatible with board area reduction and deviceminiaturization.

Fifth Embodiment

As mentioned with FIG. 8, a mechanism for adjusting a clock that clocksan upstream flip-flop is required in the sending unit in order tosynchronize the output timing, in the case where one of a plurality ofinput signals is a clock signal.

FIG. 12 shows, as a fifth embodiment, a configuration of a differentialtransmission circuit board in which data signals are switched so as tohave axisymmetry with reference to a clock signal, in the case where aneven number of data signals are sent with respect to a single clocksignal.

In FIG. 12, reference numeral 1200 denotes a sending unit that sendsdifferential signals, and 1220 denotes a receiver that receives thedifferential signals sent by the sending unit 1200. Reference numerals1240 a, 1240 c and 1240 b denote transmission paths (first transmissionpaths, third transmission paths, second transmission paths) fortransferring all of the differential signals. Reference numerals 1201,1202 and 1203 denote input signals (first input signal, third inputsignal, second input signal) for generating the differential signals.Reference numerals 1211, 1212 and 1213 denote flip-flop circuits forrespectively holding the Data1, Clk and Data2 input signals 1201, 1202and 1203. Reference numerals 1214, 1215 and 1216 denote buffers forrespectively generating the differential signals (first differentialsignals, third differential signals, second differential signals) fromthe input signals 1201, 1202 and 1203. Reference numerals 1230, 1231 and1232 denote buffers that respectively receive the differential signalssent via the transmission paths 1240 a, 1240 c and 1240 b, and outputdifferences (first differential signal, third differential signal,second differential signal). Reference numerals 1233, 1250 and 1234denote inverters for respectively inverting the polarities of signalsfrom the buffers 1230, 1231 and 1232. Reference numerals 1235, 1251 and1236 denote selectors (first selector, fifth selector, second selector)for respectively selecting whether to pass the signals from the buffers1230, 1231 and 1232 through the inverters 1233, 1250 and 1234. Referencenumerals 1237 and 1238 denote selectors (third selector, fourthselector) for switching the signals selected by the selectors 1235 and1236 as necessary. Reference numerals 1221 and 1223 respectively denoteoutput signals generated by the selectors 1237 and 1238, and 1222denotes an output signal obtained by doubling the frequency of thesignal from the selector 1251. Reference numerals 1224 and 1226 denoteflip-flop circuits for respectively holding the output signals 1221 and1223. Reference numeral 1225 denotes a PLL (Phase Locked Loop) circuitfor multiplying the signal frequency.

Next, the case where the sending unit and the receiver are implementedon the same surface of the board will be described.

Differential signals are generated by the buffers 1214, 1215 and 1216from the input signals 1201, 1202 and 1203 and sent to the receiver 1220via the transmission paths 1240 a, 1240 c and 1240 b. The selectors 1235and 1236 select the signals generated by the buffers 1230 and 1232 fromthe received differential signals, and the selected signals are selectedas the output signals 1221 and 1223 by the selectors 1237 and 1238without being switched. The output signals 1221 and 1223 arerespectively held in the flip-flop circuits 1224 and 1226. The frequencyof the signal generated by the buffer 1231 from received differentialsignals and selected by the selector 1251 is multiplied in the PLLcircuit 1225, and becomes the clock of the flip-flop circuits 1224 and1226 as a Clk output signal 1222.

The appearance of a circuit board when the sending unit and the receiverare implemented on different surfaces of the board is shown in FIG. 13.

In FIG. 13, reference numeral 1380 denotes a circuit board, and 1381denotes via holes. According to this configuration, the polarities ofbuffers 1332, 1331 and 1330 are switched in order to keep transmissionpaths 1340 a, 1340 c and 1340 b parallel and isometric. Differentialsignals are generated by buffers 1314, 1315 and 1316 from input signals1301, 1302 and 1303, and sent to a receiver 1320 via the transmissionpaths 1340 a, 1340 c and 1340 b.

Selectors 1335, 1351 and 1336 respectively select signals generated inbuffers 1332, 1331 and 1330 from the differential signals received viathe transmission paths 1340 a, 1340 c and 1340 b and polarity invertedin inverters 1333, 1350 and 1334. A selector 1337 selects the signalfrom the selector 1336, and a selector 1338 selects the signal from theselector 1335, and the selected signals are respectively held inflip-flop circuits 1326 and 1324 as output signals 1323 and 1321. Thefrequency of the signal generated in the buffer 1331 from thedifferential signals received via the transmission paths 1340 c andselected by the selector 1351 is multiplied in the PLL circuit 1325, andbecomes the clock signal of the flip-flop circuits 1324 and 1326 as aClk output signal 1322.

Here, while the case where two Data signals and a single Clk signal aretransmitted from the sending unit and the Data signals are switched inthe receiver has been described, the present invention is alsoapplicable in the case where four or more Data signals are transmittedor where the Data signals are switched in the sending unit.

According to the present embodiment, a differential transmission circuitthat is compatible in the case where the sending unit and the receiverare implemented either on the same surface or on different surfaces of aboard with IC chips having the same terminal arrangement can be realizedwhen transmitting an even number of data signals with respect to asingle clock signal using a differential transmission system. Thisdifferential transmission circuit is also compatible with board areareduction and device miniaturization.

Sixth Embodiment

FIG. 19 shows a configuration of a differential transmission circuitboard of a sixth embodiment according to the present invention. In FIG.19, reference numeral 1900 denotes a sending unit that generatesdifferential signals from a prescribed input signal, and sends thegenerated differential signals. Reference numeral 1910 denotes areceiver that receives the differential signals sent by the sending unit1900, and generates an output signal. Reference numerals 1901 and 1902are respectively a P terminal and an N terminal provided in the sendingunit 1900, and reference numerals 1911 and 1912 are respectively a Pterminal and an N terminal provided in the receiver 1910. Referencenumerals 1920 a and 1920 b denote a first transmission path and a secondtransmission path that transfer the differential signals. Referencenumeral 1980 denotes a circuit board.

The P terminal 1901 and the N terminal 1902 of the sending unit 1900 andthe P terminal 1911 and the N terminal 1912 of the receiver 1910 arethus lined up in the same direction as the first transmission path 1920a and the second transmission path 1920 b. That is, a straight lineconnecting the P terminal 1901 and the N terminal 1902 of the sendingunit 1900 and the P terminal 1911 and the N terminal 1912 of thereceiver 1910 is parallel to the first transmission path 1920 a and thesecond transmission path 1920 b. The transmission paths 1920 a and 1920b are bent within the respective packages of the sending unit 1900 andthe receiver 1910, so that the transmission paths 1920 a and 1920 b arekept parallel at a prescribed interval. Note that while the effects ofthe present invention are obtained even without bending the transmissionpaths, an extra via hole is needed in this case in order to prevent thetransmission paths from coming into contact with one another.Transmission paths are susceptible to the effects of neighboring signalsas a result of passing through internal layers of a board, but thesedrawbacks can be avoided by bending the transmission paths.

The appearance of a circuit board when a sending unit 2000 and areceiver 2010 are implemented on different surfaces of a board is shownin FIG. 20. In FIG. 20, reference numeral 2080 denotes a circuit board,and 2081 denotes via holes. The dotted lines show that the receiver isimplemented on the surface on the opposite side of the board 2080.

A straight line connecting the four terminals, namely, a P terminal 2001and an N terminal 2002 of the sending unit 2000 and a P terminal 2011and an N terminal 2012 of the receiver 2010 are parallel to thetransmission paths 2020, and also the P terminal 2001 and the N terminal2002 of the sending unit 2000 and the P terminal 2011 and the N terminal2012 of the receiver 2010 are lined up equidistant in the samedirection, parallel to the transmission paths 2020. The transmissionpaths thereby do not intersect even in the case where the sending unit1900 and the receiver 1910 are implemented on the same surface of theboard 1980 as shown in FIG. 19, or where the sending unit and thereceiver are implemented on different surfaces as shown in FIG. 20.Accordingly, the transmission paths can be kept isometric and parallel,and differential transmission without loss of signal quality ispossible.

FIG. 21 shows the case where a P terminal 2101 and an N terminal 2102 ofa sending unit 2100 and a P terminal 2111 and an N terminal 2112 of areceiver 2110 are arranged so as to be lined up in different directions,parallel to transmission paths 2120. Reference numeral 2180 denotes acircuit board, and 2181 denotes via holes. The dotted lines show thatthe receiver is implemented on the surface on the opposite side of aboard 2180.

FIG. 22 shows the case where a straight line connecting a P terminal2201 and an N terminal 2202 of a sending unit 2200 and a straight lineconnecting a P terminal 2211 and an N terminal 2212 of a receiver 2210are orthogonal, and a straight line connecting the P terminal and the Nterminal of one of the sending unit 2200 and the receiver 2210 isperpendicular with respect to transmission paths 2220. Reference numeral2280 denotes a circuit board, and 2281 denotes via holes. The dottedlines show that the receiver is implemented on the surface on theopposite side of the board 2280. The transmission paths can be keptparallel in the case of both FIG. 21 and FIG. 22.

Accordingly, a differential transmission circuit board that iscompatible in the case where the sending unit and the receiver areimplemented either on the same surface or on different surfaces of aboard with IC chips having the same pin arrangement can be realized.This differential transmission circuit is also compatible with boardarea reduction and device miniaturization.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

The invention claimed is:
 1. A differential transmission circuitcomprising: a sending unit that generates a pair of differential signalsfrom an input signal, and sends the differential signals from a Pterminal and an N terminal; a receiver that receives, at a P terminaland an N terminal, the differential signals sent by the sending unit,and generates an output signal; a first transmission path that transfersa differential signal from the P terminal of the sending unit to the Pterminal of the receiver; and a second transmission path that transfersa differential signal from the N terminal of the sending unit to the Nterminal of the receiver, wherein the sending unit and the receiver arearranged on a circuit board, a straight line connecting the P terminaland the N terminal of the sending unit and a straight line connectingthe P terminal and the N terminal of the receiver are parallel, a gapbetween the P terminal and the N terminal of the sending unit is equalto a gap between the P terminal and the N terminal of the receiver areparallel, a gap between the P terminal of the sending unit and the Pterminal of the receiver is equal to a gap between the N terminal of thesending unit and the N terminal of the receiver, the first transmissionpath comprises a straight portion and a bent portion, the secondtransmission path comprises a straight portion and a bent portion, thestraight portion of the first transmission path is lined up parallel tothe straight portion of the second transmission path, the P terminal andthe N terminal of the sending unit are lined up parallel to the straightportions of the first transmission path and the second transmissionpath, the P terminal and the N terminal of the receiver are lined upparallel to the straight portions of the first transmission path and thesecond transmission path, the bent portion of the first transmissionpath extending from the P terminal of the sending unit and the bentportion of the second transmission path extending from the N terminal ofthe sending unit are extended to be directed away from each other. 2.The differential transmission circuit according to claim 1, wherein thesending unit and the receiver are arranged on surfaces on opposite sidesfrom one another with respect to the circuit board.
 3. The differentialtransmission circuit according to claim 1, wherein the sending unit andthe receiver are arranged on the same surface on the circuit board. 4.The differential transmission circuit according to claim 1, wherein alength of the first transmission path is the same as a length of thesecond transmission path.